A number of approaches have been proposed in the past for imaging systems to achieve a wide field-of-view (FOV). Most of them, however, are able to generate omni-directional images. By “omni-directional images,” we mean images with a field-of-view covering entire hemisphere (180 degrees of solid space angle), simultaneously.
Most existing imaging systems employ electronic sensor chips or still photographic film to record images collected by an optical lens system. The image projection for most camera lenses is modeled as a “pin-hole” with a single center of projection. The light rays that can be collected by a camera lens and received by an imaging device typically form a corn with very small opening angle. Therefore, angular field-of-views for conventional cameras are within a range of 5 to 50 degrees. For example, an 8.5 mm F/1.3 camera lens for a ½″ CCD (Charge Coupled Device) chip only has an angular FOV of 41.2 degrees.
Optical engineers have designed several versions of wide-viewing-angle lens systems called the fish-eye lens. The fish-eye lens features a very short focal length which, when used in place of a conventional camera lens, enables the camera to view objects over a much wider angle. In general, the wider FOV, the more complicated design the fish-eye lens has. To obtain a hemispherical FOV, the fish-eye lens must be quite large in dimension, complex in optical design, and hence expensive. Also, it is very difficult to design a fish-eye lens that meets the single viewpoint constraint, i.e., all incoming principal light rays intersect at a single point to form a fixed viewpoint.
Fish-eye lenses also introduce distortion into the images generated. Although the images acquired by fish-eye lenses may prove to be good enough for some visualization applications, the distortion compensation issue has not been resolved.
A large field of view of may also be obtained by using multiple cameras in the same system, each pointing toward a different direction. However, seamless integration of the multiple resulting images is complicated and made further difficult by the fact that the image produced by each camera has a different center of projection. The cost for such a system is usually high. The image processing required for multiple cameras or a rotating camera to obtain precise information on the position and azimuth of an object takes a long time and is not suitable for most real-time applications.
Another straightforward solution to increasing the FOV of an imaging system is to rotate the entire imaging system about its center of projection. This concept is illustrated in FIG. 1. The image sequence (100) acquired by the camera at different positions is then “stitched” together to obtain a panoramic view (101) of the scene. The first disadvantage of any rotating image system is that it requires the use of moving parts, and precision positioning devices. A more serious drawback is that such systems lack the capability of simultaneously acquiring images throughout the wide FOV. Although such system can acquire precise azimuth information in omni-directional view, the imaging process is time-consuming and the method is not applicable to real-time problems such as avoiding collision against moving obstacles or monitoring scenes with mobile objects. This restricts the use of rotating systems to static and non-real-time applications.
U.S. Pat. No. 6,304,285 discloses and claims an omni-directional imaging system that overcomes these limitations on prior systems. The omni-directional camera of U.S. Pat. No. 6,304,285 is capable of capturing real-time omni-directional images without using any moving parts. FIG. 2 provides a comparison between the FOV of the omni-directional camera of U.S. Pat. No. 6,304,285, a panoramic camera and a conventional camera.
While the omni-directional camera of U.S. Pat. No. 6,304,285 provides a simultaneous FOV over 180 degrees of solid space with no moving parts, the resolution at any particular point in the resulting image may not be sufficient for all applications, including surveillance systems.